Table Of ContentInterpretation
of Metallographic Structures
THIRD EDITION
WILLIAM ROSTOKER
Rostoker, Inc.
Burham, Illinois
JAMES R. DVORAK
Fond du Lac, Wisconsin
ACADEMIC PRESS, INC.
Harcourt Brace Jovanovich, Publishers
San Diego New York Boston
London Sydney Tokyo Toronto
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Library of Congress Cataloging-in-Publication Data
Rostoker, William
Interpretation of metallographic structures / William Rostoker,
James R. Dvorak. - 3rd ed.
p. cm.
Includes bibliographical references.
ISBN 0-12-598255-0 (alk. paper)
1. Metallography. I. Dvorak, James R. II. Title
TN690.R75 1990
669'.95-dc20 89-18139
CIP
Printed in the United States of America
90 91 92 93 9 8 7 6 5 4 3 2 1
Preface to Third Edition
Light optical microscopy continues to be the main instrument and
technique of metallographic examination. At least, it should be the
first reconnoiter. Etched microstructures are now also commonly
examined by the scanning electron microscope (SEM). The appear
ances by either light or electron optical means are much the same.
The major differences are in the qualities of contrasts between
adjacent phases and at interfaces. In these, light microscopy is more
versatile, but the SEM has the advantage of being able to "zoom" to
magnifications well in excess of light optical limits.
There are a good many other instruments that have become more
widely used to supplement the optical microscope. The "quan-
tometer," or electronic area counting instrument, can now easily
provide the volume proportions of a distributed phase. The micro-
hardness tester can indicate differences in state that have not been
brought out by etching contrasts. We also add to these the variety of
instruments that can provide qualitative and quantitative analysis of
the elements present.
It is the microbeam analysis of phases and designated zones in a
microstructure that has grown so much in use since the previous
edition was published. This book is substantially augmented by the
addition of a new chapter that deals with this subject and, in particu
lar, the uses to which it may be put.
As a final word, we wish to dedicate this third edition to the
memory of a good friend and colleague, Robert F. Domagala.
ix
Preface to Second Edition
The preparation of this second edition has afforded the opportunity
of replacing a number of the specimen microstructures with more
appropriate alternatives and, of course, to correct errors in the text
that became apparent only after the first edition was published.
A major change in the text has been to treat the subject of fracture
in a separate and distinct chapter. The recognition of fracture modes
has developed considerably in the past ten years. In particular, the
scanning electron microscope has created appearance identities that
are often unique and definitive. Accordingly, fracture analysis must
use SEM and light optical microscopy jointly. This joint use is the
theme of the new chapter.
The subject of quantitative metallography has become very sophis
ticated, but the application of measurements to property and process
correlation has remained rather elementary. Opportunity has been
taken in this edition to present some of the types of correlation that
have become very useful.
xi
Preface to First Edition
While the modern metallurgist has many investigative techniques
at his disposal, the optical reflection microscope remains his most
effective means for examining the structure of metals. At the lowest
level of use, it provides a ready means for comparison and identifica
tion. With skill and experience there is always much more to be gained.
The recognition of significant features combined with an appreciation
of physical metallurgy provides a powerful basis for rationalization
and diagnosis. To teach this art requires carefully selected examples
placed within the context of real issues. This book is intended to be
such a vehicle. It is directed toward the senior student as a preview
of the scope of his subject and to the practicing metallurgist as a
reintroduction.
The book is an organized presentation of specimen microstructures,
each chosen for its clarity of illustration and each or in groups forming
the pretext for some discussion of interrelation between physical metal
lurgy and metallography. There has been no intent to produce an
encyclopedia of microstructures in the sense that all variations of all
alloy systems are catalogued. The objective has been, instead, to choose
structures characteristic in a physical metallurgy sense with the pur
pose of demonstrating that logical framework of interpretation can
supplant mental storage of infinite variations.
Discussion has been divided into two parts. Prefacing each topic is
a brief summary of those aspects of the subject which are relevant to
metallographic observation. The selection of micrographs is integrated
into this summary. Each micrograph is given an individual commen
tary in which the composition and thermal or thermal-mechanical
history are related to the particular point which is intended for illus
tration. These short descriptions frequently contain remarks which are
apropos of the structure but not necessarily of the topic under dis
cussion. This is deliberately done to encourage the reader to see more
than he is looking for.
The final chapter deviates from those preceding by approaching
the subject of quantitative metallography. This subject has matured
greatly in recent years and quantitative measurements will assuredly
xiii
xiv Preface to First Edition
become an increasingly important tool in physical metallurgy. Whether
or not the reader will have occasion to use these techniques, an under
standing of them will help in appreciating the limitations of planar
section views as representative of the structure of opaque bodies.
To demonstrate that metallographic interpretation is a common
denominator of all metallic materials, specimen microstructures have
been deliberately taken from a wide diversity of materials. This may
appear to have been carried to an extreme but the authors have felt
that great rhetorical value derives from the mixture of common and
uncommon.
The microstructures themselves are intended to be technically good
and at magnifications consistent with the objectives of the associated
text. The quality of presentation has been something of a dilemma.
One cannot forestall the reader from congratulating himself on having
seen or even prepared specimens esthetically superior to one or
another micrograph offered in the text. The authors can only hope
that each reader will actually find such occasion. On the other hand
it may be said that the presentations are not typical of what the
metallurgist most often encounters. This is undoubtedly true and the
novice diagnostician using this book as a guide may be disillusioned
on occasions of encountering the real-life problems of professional
service. At this point, the authors must reiterate their objectives of
attempting to teach rather than to catalogue.
The written presentation has attempted to embody contemporary
concepts in physical metallurgy without recreating their origin or
derivation. For these the reader must turn to the many advanced books
on the subject. Occasionally, the text makes footnote references to
certain publications which give pertinent elaboration. For more com
plete details of special techniques, standards, and general information
on metallographic technology, a classified list of suitable references
is given in the Appendix.
Introduction
The early training of a metallurgist is usually built around metal
lographic structures. The principles of phase constitution, transforma
tions and thermomechanical histories are most easily described to the
student in terms of the appearance of metals under the microscope.
Perhaps just because it is so implicit in the training of a metallurgist,
metallography is easy to regard as an established art. Yet it can be no
more settled or established than the science of metallurgy itself. For
practical reasons as well as visual appreciation, each new increment
of knowledge should be translated into the nature of the structure of
metals.
While metallography remains a versatile tool, modern metallurgy
makes use of a wide variety of physical and mechanical property and
radiation interactive measurements in the definition of states of aggre
gation. In sober appraisal there is little room for controversies on the
relative merits of various tools. The fact stands out that no tool is
self-sufficient, for they each describe different aspects of the nature of
a material—aspects whose separate knowledge is inadequate but in
joint appraisal may permit unambiguous interpretation. It remains,
therefore, to identify properly the limitations as well as the capabilities
of the tools of metallurgical study.
Since this book concerns itself with metallography as a metallurgical
tool, it behooves the reader to consider the nature of what may be
observed with the aid of an optical reflection microscope. The metal
lographic structure reveals the multicrystallite aggregation of a phase
or of a mixture of phases. This aggregation may be described in terms
of the number of phases, the morphology of the phases, and the con
figuration of phases. The microstructure cannot of itself give informa
tion on the chemistry of phases or on their crystal structure. Such
requirements need the assistance of other measurement systems.
This book will not concern itself with techniques of preparation of
a metallic specimen for examination. It is assumed that the reader is
familiar with the necessity of creating a flat and highly reflective
surface, free of observable physical imperfections and unchanged in
1
2 Introduction
nature by the method of preparation. The resolution of phases and
grain boundaries is achieved through the action of etching, most
commonly by aqueous solutions. The choice of etching reagent is
still in an empirical state, but the qualifications of such can be laid
down.
Etching is fundamentally a highly anisotropic dissolution process.
It is required that an etching solution attack each type of phase
present at significantly different rates. In this way each phase is
characterized in terms of light reflectivity by a consistent appearance
which is somewhere in the spectrum between shiny and black. Not
only must the rate of chemical attack be significantly different among
the phases present in a structure, but there should also be an observa
ble difference between the various grain or crystallite orientations
revealed by a horizontal section. Alternative to this, the rate of etch
ing must be sensitive to small changes in chemistry between the
centers of grains and interfaces or boundaries between grains. All of
this is meant to imply that the initially flat and uniformly reflective
surface must be modified to one which is a series of plateaus, the
surface of each having a roughness characteristic of phase identity
and the interfaces between represented by sharp, deep, but narrow
trenches. A better etchant is one which gives sharper resolution to
these differences by local chemical dissolution. Often one etchant is
insufficient to distinguish all features. The etchant which makes for
sharp contrast between chemically dissimilar phases may be quite
incapable of resolving orientation variations in a given phase region.
An alternative means of phase identification is the process whereby
the surface is oxidized in chemical solution at a rate characteristic of
the phase species. The spectrum of reflectivities in this case is sup
planted by a spectrum of colors, whence the term "stain" etching. The
color spectrum itself is the product of the relationship between light
interference and oxide film thickness, and the structure produced is
not fundamentally different from the well-known temper colors en
countered in the heat treatment of steel. As with rate of chemical
attack, the degrees of stain coloration (or oxidation) must be such
as to distinguish variations in one phase type from variations in
another. This has not always been adequate because changes in
orientation can produce as large differences in coloration as changes
in chemistry.
There are still further techniques involving elevated temperature
differential oxidation or evaporation. There are also occasions when
natural light reflection differences or relief polishing are adequate. The
Introduction 3
use of polarized light is often valuable where certain phases have
optical anisotropy.
It must always be appreciated that metallographic examination
constitutes simply a planar section view of a three-dimensional struc
ture. It is not enough to recognize this fact; one must also understand
how shape in a three-dimensional construction can degenerate into
traces in random planar section. In effect, one must be able by mental-
visual skill to recreate from slices of hard-boiled egg, the oblate
ellipsoid whence they came.
Our increased ability to interpret the structures which appear
before us under the microscope reflects our increased knowledge of the
nature of liquid and solid state transformations. At least in a qualita
tive sense there is good understanding of nucleation and growth
processes, of diffusion and diffusionless processes, of the anisotropy of
growth, of the influences of temperature and time on transitions from
metastability to stability. In such studies, metallography has been a
necessary tool. As with many of our tools of study, their use and
accomplishments have in turn enhanced their own capability.
Perhaps more than anything else, the understanding of the role of
surface or interfacial energy in growth and transformation processes
has broadened our power of interpretation. Interfacial energies domi
nate the transitions from metastable to stable states and the mor
phologies and configurations of phases which develop.
By developing interpretive skill, one can attempt to rationalize the
significance of a new and unfamiliar structure. But this must be done
within the context of all available information. Metallographic in
terpretation is not a self-sufficient process. It must utilize what is
available in terms of composition, established phase equilibria, process
history, and service experience, as well as what can be made available
in terms of physical property measurements, microprobe analysis, and
structural studies.
In any system of interpretation, it is the generalizations that are
most important. Structures encountered in brasses have counterparts
in ferrous and other alloy systems. The transformation of austenite to
pearlite in carbon steel is not fundamentally different from the de
composition of the MgZn« phase to Mg terminal solid solution and
7
MgZn. The kinetics may be different but the variations of morphology
and the derivative processes are the same.
CHAPTER I
Polycrystalline Structures
Equiaxed, Single Phase, Polycrystalline Grain Structure
The microstructure of Fig. 1.1 portrays a polycrystalline assembly
of single phase species. As such it represents a planar cross section of
an aggregate of single crystals as in Fig. 1.2 fitting together to fill
space completely. The polygonal shapes of the individual grains in
the microstructure illustrate versions of idealized solid shapes which
concurrently satisfy requirements to fill space completely, conform to
topological restrictions, and balance interfacial tensions at intersections.
Any large and representative sample of fitting polygons in two-
dimensional arrangement must conform to a topological relationship
involving the number of polygons, P, and the number of sides on each
polygon, n.
£(6-n)P = 6°
n
A test of conformity to this rule serves to establish the adequacy of
the sample. A similar relationship holds for an assembly of fitting
polyhedra of number, B, having polygonal faces, P, each having a
n
number of sides, n.
2(6 - n)P = 6(B + If
n
Note that in this scheme, there is no necessity for the polygons and
polyhedra to be of identical size or shape.
But the grains of a metallurgical material are not simple geometri
cal arrangements. The bounding faces between crystallites have
surface or interfacial energies. Except for low angle and twin relation
ships, the interfacial energies of randomly oriented grains are all
nearly of the same magnitude. This means that along a line of inter
section of three adjacent grains, surface tension vectors in each inter-
facial plane and hence the planes themselves in the immediate vicinity
* C. S. Smith, "Metal Interfaces," pp. 65-113. ASM, Metals Park, Novelty,
Ohio, 1952.
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